Syk and the Src-family tyrosine kinases are critical signaling components of the immune response and other fundamental cellular processes including proliferation, survival, and adhesion. Syk and Src-family kinases work together to regulate immune signaling. Deregulation of the cellular phosphorylation catalyzed by these kinases causes a vast number of autoimmune disorders and cancer. Fundamental to their regulation is localization through protein-protein association and activation of enzyme catalytic activity. A complete molecular understanding of these processes would elucidate control mechanisms of cell signaling that could potentially be exploited by other research efforts to design new, more selective inhibitors in drug development. This project targets two objectives. The first is to investigate the phosphorylation-dependent allosteric control of Syk interaction wit membrane receptor activation motifs and a functional difference of Zap-70 in receptor recognition. The second is to define Src kinase-substrate interactions and to investigate the mechanism of conformational activation for the purpose of gaining structural insights that could be exploited in developing selective kinase inhibitors. To achieve these objectives, the project specifically aims to determine if tyrosine phosphorylation on linker A of Syk kinases modulates Syk and Zap-70 association with receptor by an entropic mechanism and with no effect on binding contacts using NMR methods that include paramagnetic relaxation and residual dipolar couplings. The findings will advance our understanding of an inhibitory mechanism of Syk kinases with a particular thermodynamic signature and allosteric in nature, and differentiate the functions of Syk and Zap-70 in immune signaling. Another aim of the project will combine NMR experiments and computer simulation methods to characterize Src kinase substrate recognition and conformational dynamics related to catalytic activation. The intended outcome will be to obtain a mechanistic picture of the transition between active and inactive forms of Src kinase, which is the foundation of regulating Src function.